`is-stopped` not bound?

According to

https://pyret.org/docs/latest/reactors.html

there should be an `is-stopped` function available if you get the `reactors` module. However, there does not seem to be any such function; only the `.is-stopped` method on reactors seems to exist. (The other functions named in that section seem to be bound, though.)

Bug found by Evan Luo.